1. Field of the Invention
The present invention relates to an aqueous cementing composition and a method of using the same in cementing oil and gas wells and the like. More particularly, the present invention relates to the incorporation into the cementing composition of a blend of copolymers, as fluid loss additives which a synergism when combined into the blend. The blend includes (i) a copolymer produced by the copolymerization of acrylamide and vinyl imidazole in ratios of from about 95:5 to 5:95 by weight, and a (ii) another copolymer of vinyl pyrrolidone and the sodium salt of vinyl sulfonate in a ratio of 80:20 to 20:80 as fluid loss additives in oil and gas well cementing operations.
The fluid loss additive blend is comprised of from about 20:80 to 80:20 copolymer (i) to copolymer (ii), and preferably 80:20 copolymer (i) to copolymer (ii).
The incorporation of the blend of such copolymers as a fluid loss additives for the cement slurries used in the completion of oil and gas wells, greatly reduces the loss of water from the cement slurry to the rock formation. Indeed, a synergism is observed which is greatly unexpected when one considers the fluid loss control exhibited by each copolymer separately.
Optionally, a dispersant such as polynaphthalene sulfonate may also be incorporated into the cement slurry with the copolymer and terpolymer blend. When polynaphthalene sulfonate is incorporated into the slurry with the fluid loss additive, there is an added effect between the dispersant and the additive blend which results in an even greater reduction of fluid loss than could be achieved using either ingredient alone.
The incorporation of the fluid loss additives blends of the present invention into cement slurries greatly reduces fluid loss in oil and gas well cementing operations and allows for a more efficient bonding between the oil or gas well liners and the rock formations.
2. Description of the Prior Art
Oil well cementing and other underground cementing operations often require placement of a slurry of cement, water and other additives in a porous environment such as porous earthen or rock strata. For example, cement compositions are routinely used in the oil and gas industry to cement the annular space in the well bore between the surrounding formation and the pipe or casing. Typically, the cement slurry is pumped down the inside of the casing and back up the outside of the casing through the annular space. The slurry is allowed to set up or harden in the annular space, thereby forming a rigid column which ideally forms a bond with the earth or rock formations as well as with the metal pipe or casing. To achieve a satisfactory primary cementing job, it is essential to achieve a tight bond to prevent vertical communication of fluids or gas along or within the column, which could contaminate the producing zone, or permit a loss of reserves.
The primary functions of the cementing process are to restrict fluid movement between geological formations and to bond and support the casing or metal pipe. In addition, the cement aids in protecting the casing from corrosion, preventing "blow-outs" by quickly sealing formations, protecting the casing from shock loads in drilling deeper wells and sealing off lost circulation or thief zones.
A common problem in petroleum well cementing is the flow of liquid from the slurry into the porous earth formations in contact with the cement. This fluid loss is undesirable since it causes thick filter cakes of cement solids which can ultimately plug the well bore. The fluid loss can also damage rock formations and affect well production. Fluid loss from cement slurries is particularly a problem in a process known as "squeeze cementing".
Problems develop when water filters out of the slurry and into the porous media during the placement and setting period of the cement. As a result of the attendant rapid water loss, the cement experiences impaired qualities of strength and an uncontrolled setting rate. Also, the water loss from the cement frequently damages the porous rock formations. This problem cannot be solved by adding more water to the slurry as this approach only exacerbates the problem. In addition, serious placing and setting problems may occur.
It is therefore of utmost importance that fluid loss control be achieved in order to insure satisfactory primary cementing. Inadequate fluid loss control can result in the formation of a bridge in the annulus opposite a permeable zone, thus isolating a lower zone from the hydrostatic pressure above the bridge. Only a small amount of filtrate loss beneath such a bridge is then necessary to drop the annular pressure to beneath that of the formation. The result is an influx of formation fluids and pressure, thereby creating flow channels and the need for often times expensive remedial work.
In order to attempt the control of fluid loss from the cement slurry to the surrounding rock formation, the cement matrix permeability must be reduced. This reduction allows the retention of a greater amount of water during the initial set, thereby effectively blocking the porous strata from the cement. The art is replete with examples of methods to achieve this goal. One way is to reduce filtrate mobility by increasing the filtrate viscosity to counter the normal thinning of the cement slurry which occurs at down hole temperatures. An increase in filtrate viscosity at down hole temperatures minimizes thermal thinning and increases the retention of the filtrate within the cement matrix. Conventional fluid loss additives do not satisfactorily address the problem of thermal thinning with increased temperature, thereby allowing increased fluid loss from the slurry to the formation and promotion of stratification of solids within the cement slurry column.
Accordingly, there is a greatly felt need for new materials which, when added to the cement slurries, reduce fluid loss to the surrounding rock formations.
Fluid loss additives in cementing compositions are old and well known in the art. Fluid loss additives have been discussed in the following articles:
Carter, Gregg and Slagle, Knox, "A Study of Completion Practices to Minimize Gas Communication", Society of Petroleum Engineers, Paper No. 3164, November 1970.
Christian, W. W., Chatterji, Jiten and Ostroot, Warren, "Gas Leakage in Primary Cementing--A Field Study and Laboratory Investigation", Society of Petroleum Engineers, Paper No. 5517, October, 1975.
Cook, C. Cunningham, W., "Filtrate Control: A Key in Successful Cementing Practices", Journal of Petroleum Technology, August, 1977, page 951.
Smith, Dwight, "Cementing: SPE Monograph Volume 4, published by Millet the Printer, Inc., Dallas, Texas, 1976.
The patent literature is also replete with many attempts to overcome the fluid loss control problems associated with oil and gas well cementing operations. There are many references directed to protecting potable water by isolating hydrocarbon bearing strata by efficient cementing operations.
Uhl, U.S. Pat. No. 4,471,097 relate to auxiliary agents for chemical flooding of petroleum deposits and auxiliary agents used in well drilling fluids. These agents are water-soluble copolymers containing 20 to 80 percent by weight of unsaturated olefinic sulfonic acid, 5 to 15 percent by weight vinylacylamine, 0 to 40 percent by weight acrylamide and/or methacrylamide, 5 to 50 percent by weight vinylimidazole, 0 to 10 percent by weight of ##STR1## wherein R.sup.5 is hydrogen or methyl, and R.sup.6 represents hydroxy, alkoxycarbonyl with 1 to 12 carbon atoms in alkoxy moiety, cycloalkoxycarbonyl with 6 to 10 carbon atoms in cycloalkoxy moiety, phenyl, alkanoyloxy with 1 to 4 carbon atoms, or .beta.-hydroxyalkoxycarbonyl with 2 or 3 carbon atoms in hydroxyalkoxy moiety; and 0 to 25 percent by weight of a crosslinking agent containing at least two olefinic double bonds.
These copolymers are used in drilling fluid additives during drilling operations. WP 8302449, which is the equivalent of U.S. Pat. No. 4,471,097 discloses the use of these copolymers in deep bore cement compositions to act as rheology additives.
No showing is made in Uhl et al of using these copolymers as fluid loss additives in cement slurries to avoid fluid loss from the cement to surrounding rock formations, and without adversely affecting the viscosity of the cement slurry.
Siegle, U.S. Pat. No. 3,197,428 discloses compositions comprising cement and copolymers of acrylamide and acrylic acid to improve well cementing operations and reduce fluid loss from the cement to the rock formations. However, the compositions of Siegle are not entirely satisfactory because they retard cement setting at high temperatures and so cannot be used at elevated temperatures and pressures such as are encountered in deep oil and gas well operations.
Weisend, U.S. Pat. No. 3,359,225 discloses cement additives containing polyvinylpyrrolidones and a condensate of sodium naphthalene sulfonate and formaldehyde. The polyvinylpyrrolidone reduces the separation of water from the cement slurry. The naphthalene sulfonate condensate is the dispersant. There is no teaching of the copolymers and/or terpolymers of the present invention.
Gibson et al, U.S. Pat. No. 3,491,040 disclose an aqueous hydraulic cement slurry including hydraulic cement, water, a surfactant and a small amount of polyalkylene polyamine, polyalkenimine or mixtures thereof. Gibson et al also disclose a sulfonated naphthalene condensate dispersant as an additional additive to the cement slurry which cooperates with the polyamine additive to provide satisfactory fluid loss in cement slurries used at about 200.degree. F. and below. The sulfonated naphthalene dispersant is typically a low molecular weight material, e.g., in the range of about 1,000 to 3,000.
Harrison, U.S. Pat. No. 3,409,080 discloses an aqueous cementing composition which is adapted to high turbulent flow. The disclosure teaches the polyvinyl alcohol and polyvinyl acetate can be used as fluid loss additives in oil well cements.
Perisinski et al, U.S. Pat. No. 4,015,991, discloses a fluid loss additive for cement compositions which is a copolymer of acrylamide/2-acrylamido-2-methylpropane sulfonic acid derivative. These copolymers are useful only in operations where the bottom hole circulation temperature ranges from 90.degree. to 125.degree. F. Further, these copolymers have a salt tolerance of only up to 10 percent.
Cellulose-based fluid loss additives such as methyl cellulose, carboxymethylcellulose (CMC) and hydroxyethylcellulose (HEC) may be employed with or without a dispersant such as polynaphthalenesulfonic acid salts. However, there are several disadvantages to the use of CMC or HEC as cement fluid loss control additives. These materials are solid and as a result are difficult to handle in offshore operations. In addition, they tend to considerably increase slurry viscosity, thereby preventing its movement under turbulent flow conditions and retard the set of the cement. Also, these materials lose effectiveness in the presence of soluble calcium salts and at elevated temperatures.
Hence, the industry desires a fluid loss additive that has as little effect on cement properties as is possible and still provides for the fluid loss properties which are necessary for the cementing of casings to rock formations. Further, any fluid loss additives should be compatible with as many other additives as possible and should be usable over as wide a range of temperatures and other environmental conditions as is possible.